Currently all wellhead systems comprise a rigid extension, normally of steel, welded onto a shaped lower part of the wellhead body. Such an extension is exposed to an injected column of cement in an annular space between the extension, and any casing string that it supports, and an outer conductor housing and any casing string that it supports.
Wellhead systems are exposed to cyclic forces from, usually, the drilling rig, the marine riser, motion of the blow-out preventer (BOP) and from other causes of pressures in the well bore. Repeated forces will, if sufficiently large or extended over sufficient time, produce a liability to fatigue damage and the possible failure of the wellhead system. The aforementioned column in the annular space is important to ensure control of the well. However, there is no established means of controlling the final height of the cement and there is a presumption that the cement will extend up to the level of the circulation ports. Cement at this high level on the outside of the wellhead extension reduces the freedom of the extension to flex. This loss of freedom is liable to produce repetitive stress which is potentially great enough to cause fatigue failure.